McDowell said the probability is low that a large lump of toxic stuff will prove hazardous.
He noted that some of the probe's equipment is dense and could survive re-entry, but added the odds are that any surviving pieces will wind up in the ocean.
"All the best rules in the world" put in place to prevent uncontrolled satellites from crashing down do little if any good in the event of a launch failure, McDowell said. "This is always going to be the risk that something breaks, and you end up with a situation like this. You can minimize it, but you can't prevent it entirely."
The $170-million Phobos-Ground mission was Russia's most expensive and the most ambitious space endeavor since Soviet times. The spacecraft was intended to land on the crater-dented, potato-shaped Martian moon, collect soil samples and fly them back to Earth, giving scientists precious materials that could shed more light on the genesis of the solar system.
The probe was successfully launched Nov. 9 and entered a preliminary orbit where its engines were supposed to fire to set it on its path to Mars. They never did, and attempts to fix the glitch by Russian and European Space Agency experts failed.
Russia's space chief has acknowledged the Phobos-Ground mission was ill-prepared and risks of its failure were high, but said that Roscosmos had to give it the go-ahead so as not to miss the limited Earth-to-Mars launch window.
Phobos-Ground marked Russia's first planned foray beyond Earth's orbit since a botched 1996 robotic mission to Mars. That probe, designed by the same Lavochkin company, crashed shortly after launch due to an engine failure. The firm also built two other Phobos-bound probes that failed in 1988.
The crash of Mars-96 generated strong international fears because of some 200 grams of plutonium onboard. The craft eventually showered its fragments over the Chile-Bolivia border in the Andes Mountains, and the pieces were never recovered.
Russian officials continue to insist the craft plunged into the Pacific, their way of deflecting criticism for not warning the inhabitants of the impact area and for failing to search for plutonium and other debris.
Fears of radiation also were sparked by the fall of a nuclear-powered Soviet spy satellite that crashed over northwestern Canada in January 1978. The Soviets claimed the craft completely burned on re-entry, but a massive recovery effort by Canadian authorities recovered a dozen fragments, most of which were radioactive.
The Phobos-Ground contains a tiny quantity of the radioactive metal Cobalt-57 in one of its instruments, but Roscosmos said it poses no threat of radioactive contamination.